The invention relates to a display screen, particularly a liquid crystal display screen, with a backlight, comprising at least a light source and an optical guide plate having a front face facing the display screen, a back face and side faces. The invention also relates to an optical guide plate having a front face, a back face and side faces.
Liquid crystal display screens are passive display systems, i.e. they do not emit light themselves. These display screens are based on the principle that light passes or does not pass the layer of liquid crystals. This means that an external light source is required so as to generate an image. In reflective liquid crystal display screens, the ambient light is used as an external light source. In transmissive liquid crystal display screens, artificial light is generated in a backlight system.
A backlight system comprises a light source which supplies light for background illumination, and an optical guide plate having a front face, a back face and a plurality of side faces. One side face of the optical guide plate, which is adjacent to the light source, receives light generated by this source so as to guide it as background illumination to the liquid crystal display screen. Further components of the backlight system are a reflection layer which reflects light exiting from the back face back into the optical guide plate, and light-scattering elements which are present inside the optical guide plate.
When a light ray emitted by the light source enters the optical guide plate, it normally travels in the longitudinal direction of the light source and does this on the basis of repetitive, internal total reflection on the front and back face. Then the light ray is incident on the scattering material, the condition for an internal total reflection is not met and the path in the longitudinal direction is interrupted. A part of the light is scattered to the liquid crystal display screen so as to ensure its background illumination.
Different principles are used for light scattering. For example, an optical guide plate having notches in the front and/or back face is known from EP 0 590 511 A1. For example, an optical guide plate comprising air bubbles of a given size is described in patent abstracts of Japan, publication no. 08248232A.
In practice, it has proved to be difficult to determine the spatial arrangement of these scattering structures in such a way that a possibly homogeneous luminance occurs in the plane of the front face. This particularly applies when using more or less punctiform light sources such as, for example, light-emitting diodes. To achieve a homogeneous distribution, a plurality of light-emitting diodes must be used.
It is an object of the invention to provide a backlight system for display screens with an improved homogeneity of the light distribution.
This object is solved by a display screen with a backlight, comprising at least a light source and an optical guide plate having a front face facing the display screen, a back face and side faces, and is characterized in that, for reflection of a light ray emitted by the light source with respect to its vertical components and for diffuse scattering with respect to its other components, the optical guide plate has reflective indentations in at least one side face and/or recesses having vertical walls.
As a consequence of the indentations in the side faces, or the recesses in the optical guide plate, light rays are randomly distributed in the xy plane of the optical guide plate, while the z component is maintained during reflection. The incident light is therefore uniformly spread across the optical guide plate.
It is preferred to provide the reflective indentations regularly and along all side faces.
Since the reflection on the side faces of the optical guide plate does not violate the condition for the total reflection of the front and the back face, the light can be effectively spread across the optical guide plate by reflection on the edges of the optical guide plate.
It may be preferred to provide the recesses perpendicularly to the front and/or back face.
The recesses having vertical walls also improve the homogeneity of the light distribution in the optical guide plate. When a light ray is incident on the wall of such a recess, it is totally reflected either on the lateral face or the upper boundary of the recess, or it is refracted into the recess. After the light ray has been refracted into the recess, it can leave this recess through the perpendicular lateral face, while the z component of the light ray is maintained and the xy direction is changed due to refraction. The light is homogeneously spread across the xy plane of the optical guide plate.
It is also preferred that the recesses penetrate the optical guide plate completely or partly.
By means of the recesses, the light can be better coupled out into the direction of the display screen. When a light ray is incident on the wall of such a recess, it is totally reflected either on the lateral face or the upper boundary of the recess, or it is refracted into the recess. When the light ray exits through the upper or lower boundary of the recess, the light ray is generally scattered out of the optical guide plate. The magnitude of this effect and the angle distribution of the coupled-out light are directly dependent on the ratio between the height of the recess and its cross-section and can thus be well controlled.
It may be preferred that scattering elements are present on the bottoms of the recesses.
An additional distribution of the light rays can be achieved by means of the scattering elements. When a light ray laterally enters a recess through the perpendicular lateral face, and when it is incident on a scattering element, it can be scattered in such a way that it exits through the upper opening of the recess or reaches the optical guide plate again through the perpendicular lateral face of the recess. In the latter case, the light rays are spread across the xy plane of the optical guide plate, while the condition for a total reflection on the front face and the back face of the optical guide plate is maintained.
The invention also relates to an optical guide plate having a front face, a back face and side faces, having reflective indentations in at least one side face and/or recesses having vertical walls.
Due to the reflective indentations in at least one side face and/or the recesses with vertical walls, incident light rays are reflected with respect to the vertical component of the optical guide plate and diffusely scattered with respect to the other components of the optical guide plate. The light rays are spread across the xy plane of the optical guide plate, ensuring a homogeneous distribution of the light.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.